Yeah I figured if I'm gonna get the 2x converter for my 70-200 f4 is at home. But it would also be fun to have the 100-400 and be able to make it a 200-800. The 2x converter makes it an f/5.6 when used anyway. I would probably like to upgrade my 70-200 to the 2.8 IS first though. Such a baller lens, despite the weight.

Keep in mind that you lose AF if you put the 2x TC on the f/4, unless you have a 1-series body. The 2x converter adds 2-stops, so you won't get a f/5.6 unless you have a f/2.8 to begin with.